Air heater and method of making the same



Jan. 19, 1932. c BULKELEY 1,841,361

AI'R HEATER AND METHOD OF MAKING THE SAME Filed Nov. 14, 1928 4 sheats-Sheet 1 yer Jan. 19, 1932. c. A. BULKELEY AIR HEATER AND METHOD OFMAKING THE SAME Filed Nov. 14, 1928 4 Sheets-Sheet 2 gmxmcoi C61 4afiwang awommq S Jan. 19, 1932. Q BULKELEY 1,841,361

AIR HEATER AND METHOD OF MAKING THE SAME Filed Nov. 14. 1928 4Sheets-Sheet 4 D far D D l D c xi f?) B 17 p 3 a' Q A y 4 I 60 Y Y s. Z%.7; 67 v 7 j x J H I 6' gwmntoz J as * H H *r r T C/wd A My 2 \g 1 zz I0W1 010% aflozncqs Patented Jan. 19, 1932 UNITED STATES PATENT OFFICECLAUDE A BULl iELEY, OF KENMORE, NEW YORK, ASSIGNOR TO NIAGARA BLOWERCOMPANY, OF BUFFALO, NEW YORK, A CORPORATION OF NEW YORK AIR HEATER ANDMETHOD OF MAKING THE SAME Application filed November 14, 1928. SerialNo. 319,434.

This invention relates to an air heater and more particularly to aheater embodied in a unitary structure in which the air to be heated isdrawn over heated surfaces by a motor driven fan and subsequentlydischarged into the room, although the principles of the presentinvention can also be embodied in other analogous devices where heattransfer occurs, such as in air conditioners or the like.

One of the objects of this invention is to provide a heater assemblycomposed of tubes and headers in which a materially increased heatexchange is effected as compared with similar structures now in use,thereby permitting of employing more compact assemblies for givenrequirements and economizing in the size of the complete heater and thespace occupied by the same.

Another important object of this invention is to provide a heatercomposed of a plurality of pipes around which the air to be heated isdrawn .by a motor driven fan in which a. greater rise in temperature ofthe same amount of air passing through the coil at the same velocity iseffected without increasing the friction tothe flow of air passingthrough the coil and without increasing the face area or face dimensionsof the coil. By this means, with a demand for heaters of varyingcapacity the manufacturer can employ the identical shell, motor and fanin each heater and by varying the heating coil in accordance with thepresent invention,can provide heaters of the different capacitiesdesired. Such heaters are of identical dimensions and consequently arereceived interchangeably in the same heater casings and since they havethe same resistance to the flow of air, the same fans and motors can beused for each. It is also obvious that should the owner desire a heaterof greater capacity, it is only necessary to replace the heating coil toeffect this result.

A further aim is to provide such a coil which is composed of hair pintubes connecting the two headers in which the tubes are so formed as toprovide a proper gradient for each tube regardless of whether the tubebundle or assembly is arranged horizontally or vertically, therebypermitting of the manufacture of one coil to meet both conditions.

A further aim is to provide a hair-pin tube bundle or assembly in whichthe ends of the tubes are connected to a single tube plate and separateheaders are provided. which are separately welded to the tube plate. Bythis means a heater is provided which has the manufacturing andstructural advantages of a single tube plate, and all portions of thecontacting edges of the headers are readily accessible for properlywelding the headers to the tube plate.

Another object is to provide a plate which receives the outer bends ofthe hair-pin pipes and holds them in fixed relation to each other andalso serves as a support for a cover or box which covers these bends andprevents the flow of air past the same, the heating effect of thesebends being relatively negligible. By this means the proper spatialrelation of the pipes is maintained to secure the advantages of anincreased heat transfer for a coil of given face dimensions and the tubebundle as a whole is strengthened. The covering or box also forms asupport for that end of the bundle for slidingly supporting the coil onrails or tracks in the heater shell so that the tube bundle can bereadily slid out of the same when repairs on the same are necessary.

A further aim is to provide an enclosed heater of the characterdescribed in which the tube bundle or assembly is supported on anglebars along the sides of the shell so that upon removing the pipeconnections, the bundle can be slid out as a unit from the shell for thepurpose of repairing either the tube assem ly or the shell.

Another purpose is to provide a coil assembly, composed of a tube plate,hair-pin tubes welded to the tube plate, headers welded to the tubeplate and boxes surrounding the headers and the bends of the hair-pintubes in which each of these elements is made of aluminum. By this meansnot only is a lighter and substantially non-corroding assembly provided,but it is also materially less expensive both in cost of materials andin assembly and fabrication.

In the accompanying drawings:

Figure 1 is a vertical transverse section through the shell of a heaterembodying my invention.

Figure 2 is a side elevation thereof showing a part of the shell brokenaway and disclosing the heater bundle or assembly.

Figure 3 is a top plan view of the tube bundle or heater assembly,showing the same removed from the shell.

Figure 4 is a vertical longitudinal section through a heater shellshowing the present invention applied to a ceiling type of heater inwhich the heater shell is supported horizontally from the ceiling.

Figure 5 is a View similar to Fig. 3, showing indetail the manner inwhich the bends of the tube and the headers are enclosed to prevent theflow of air past the same, and to support the bundle and maintain theproper spatial relation of the tubes.

Figure 6 is a side elevation thereof.

Figure 7 is a vertical section, taken on line 77, Fig. 5.

Figure 8 is a view similar to Fig. 7 but showing the invention appliedto a bundle having a single row of hair-pin tubes.

Figure 9 is a vertical section through the tube plate and hairpin tubesof a tube bundle or assembly embodying my invention, a single series ofhair-pin tubes being shown and the section taken being parallel to thedirectional flow of air through the bundle, this form of the coil beingdesignated as coil 1.

Figure 10 is a horizontal section of the structure shown in Fig. 9, thesection being at right angles to the directional flow of air through thebundle.

Figure 11 is a diagrammatic end view of the structure shown in Figs. 9and 10.

Figures 12, 13 and 14 are views similar to Figs. 9, 10 and 11,respectively, and showing the invention applied to a tube bundle havingtwo staggered series of hair-pin tubes, this form of the coil designatedas coil 2.

Fig. 15 is a view similar to Fig. 11 showing a simplified form ofheating surface in which the hairpin tubes are substituted by straighttubes.

Fig. 16 is a view similar to Fig. 14 showing a simplified form ofheating surface in which the hairpin tubes are substituted by straighttubes.

Similar reference characters refer to similar parts in each of theseveral views.

In its general organization the form of heater shown in the accompanyingdrawings comprises a shell or casing which is open at its lower end, aslidingly removable heater assembly composed of hairpin tubes which mayor may not be provided with fin surfaces and which are welded to a tubeplate on the opposite sides of which separate headers with pipeconnections are welded, a fan housing in the upper part of the shell, amotor driven fan in the fan housing and an outlet through which the airis discharged into the room. The invention is shown as a plied to asingle fan unit, although it is obvious that it may also be embodied inmulti-unit heaters and may also be embodied in analogous air heating orcooling devices, such as air conditioners or the like.

The shellor casing which carries the fan and heater assembly is composedof side panels 15, end panels 16, and a top plate 17, these panels andplate being secured to angle frame bars 18, the vertical corner bars ofwhlch are extended downwardly to form legs 19. The l( wer end of theshell is open to provide an air inlet 20.

In one end wall 16, a rectangular opening is provided which is coveredby a removable cover plate 21, and on the inner sides of the side panels15, angle bars 22 are secured. These angle bars extend the full lengthof the shell and incline upwardly from the lower edge of the openingcovered by the cover plate 21 and form parallel rails for removablysupporting the heater assembly indicated generally at 23.

Suitably mounted in the upper part of the shell is a fan housing 24having two or more eyes 25 through which air is drawn from the suctionchamber 26 of the shell and discharged through an outlet 27 by a fan 28.This fan is mounted on a fan shaft 29 which is journaled in suitablebearings -30 on the shell and is driven by an electric motor 31 mountedon a suitable bracket.

The heater assembly, indicated generally at 23 is generally constructedas follows:

The numeral 32 represents an aluminum tube plate which is of rectangularform and extends substantially from one side panel 15 to the oppositeside panel and rests at its lower corners on the side rails or bars 22.In the upper and lower sides of these tube plates parallel series ofholes 33 are provided which holes receive the opposite ends of aluminumhair-pin tubes 34.

These hairpin tubes are welded to the tube plates as indicated at 35 andextend substantially to the opposite end panel 16 as indicated in Fig. 2and each leg 36 and 37 thereof may or may not be provided with analuminum fin surface, such as the aluminum spiral fins 38. The hairpintubes may be arranged to provide a single series as indicated in Figs.8-11, inclusive, or they may be arranged to form two series 60 and 61 instaggered arrangement relative to one another, as indicated in Figs. 2,5, 6, 7, and 12-14, inclusive, in the latter form the tubes of oneseries being arranged between the tubes of the other series and the twoseries being ofi'set relative to one another. This latter form providesan increased rise in temperature of the same quantity of air passingthrough the coil without increase in the face area or friction of thecoil as hereinafter described.

On the side of the tube plate 32 opposite to that from which the hairpintubes extend, an upper header 39 and a lower header 40 are welded. Theseheaders are made of cast or pressed aluminum and the upper headerencloses the ends of the upper legs 36 of the hairpin tubes and iswelded at the edges of its open side to the face of the tube sheet 32,while the lower header 4O similarly encloses the ends of the lower legs37 of the hairpin tubes. The upper header 39 is formed to provide asteam inlet nipple 41 which extends through an opening provided in thecover plate 21 and the lower header 40 is formed to provide an outlet 42which also extends through the cover plate 21. The 1nlet-41 and theoutlet 42 are adapted to be connected so that steam enters the upperheader 39, traverses the upper legs 36 of the hairpin tubes, passesaround the bends 43 thereof, through the lower legs 37 and out throughthe lower header 40 and outlet 42.

It will be noted that by providing a slngle tube plate, the ends of thetubes will be held in fixed proper relation to one another and alsoprovide a rigid and strong support for the tubes and by the provision ofseparate headers, the edges of the same can be easily welded to the tubeplate and a strong and tight joint effected.

In order to hold the outer ends of the hampin tubes in a definiterelation to one another, an aluminum plate 44 is provlded which has aplurality of vertical slots 45 conforming in spacing and number to thetubes. The bend'43 in each tube is unprovided with fins and uponcompleting the assembly of the header, tube sheet and tubes. the slottedplate 44 is slipped over the bends of the tubes so that each bend isarranged in one of the slots. Each leg of each tube is then welded atits outer side to the slotted plate as indicated at 46 in Figs. 5 and 6,and the tubes at their bends are thereby held in rigid relation to oneanother. JVhen a slotted plate 44 is made for a heater having a singleseries of hairpin tubes, such as shown in Figs. 911, only one series ofspaced slots 45 is provided in the slotted plate, as shown in Fig. 8.When, a slotted plate is made for a heater assembly in which two seriesof offset or relatively staggered hairpin tubes are employed, theslotted plate is provided with two series of slots 45 which are offsetor staggered relatively to each other and each series of slots receivingthe bends of the corresponding, series of tubes. .By this means, it isapparent that the same relative spatial relation of the legs of thehairpin tubes can be maintained throughout the length of the tube, thisspatial relation being of prime importance in the present invention ashereinafter more fully explained.

The extensions of the heater assembly beyond the tube plate 32 and theslotted plate 44 have little heating effect on the air and in order tosecure the maximum transfer of heat to the air passing through thecoils, it is necessary to prevent the passage of the air past theheaders and tube bends by the provision of suitable boxes or filler-s.As best shown in Figs. 5 and 6, the boxing oil of the space around theheaders is effected by suitably welding an aluminum box 47 to the rearside and at themargin of the tube plate 32. This box extends rearwardlyfrom the tube sheet and in the installed position of the heater assemblylies within the frame formed by the angle irons 48 around the heaterassembly removal opening which is covered by the plate 21. It thereforefollows that none of the air passing up through the shell can passbetween the tube sheet 32 and the adjacent end wall of the shell andthat the air on this side is compelled to pass through the finned legsof the tubes.

In a similar manner an aluminum box 49 is welded at its edges to therear face of the slotted plate 44, as indicated in Figs. 5-8, and in theinstalled position of the assembly, the rear wall of this box laysagainst the panel 16, as shown in Fig. 2. It will be observed that theboxes formed by the tube plate 32 and the box 47 and the box formed bythe slotted plate 44 and the box 49, besides operating to enclose therelatively non-heating parts of the assembly, also rest on the angleiron rails 22 and slidingly support the other parts of the assemblythereon. These boxes also make the assembly adequately rigid so thatupon removing the pipe connections of the heater and the cover plate.21, the entire assembly can he slid out on the side rails 22 and removedfrom the casing without danger of distorting or otherwise injuring thesame. These boxes also permit the assembly to be supported on the floorafter being removed without danger of injuring the fin surfaces on thetubes.

By making the entire heater assembly composed of the hairpin tubes andtheir fin surfacing, the tube plate, the headers, the slotted plate andthe boxes attached to the tube and slotted plates of aluminum, it hasbeen found that not only is a lighter assembly provided which issubstantially non=corroding, but also that the heater is less expensiveboth in point of the actual cost of material used and the fabricationthereof than when combinations of iron, steel. or copper, are employedin making an assembly as heretofore employed. The aluminum is veryductile which permits it to be readily formed into the desired shapes,and it is easily welded together and provides a strong joint. Moreover,by the provision of a heater made throughout of the same material, allparts of the heater have the same .coeflicient of expansion and henceinternal expansion strains on different parts of the heater assembly areeliminated.

In Fig. 4 is illustrated a heater of the ceiling type embodying thepresent invention.

In this form the shell is provided with a bottom panel 50, side panels51, a top panel 52 and an end panel 53. The shell is supported fromtheceiling by suitable hangers 54 and is left open at one end to providean air inlet opening 55. A fan housing 56 is arranged in one end of theshell and a motor driven fan 57 draws the air through the shell anddischarges it through an outlet 58 in the end wall of the shell. Aheater assembly 23, similar in all respects to the heater assembly inthe fioor type of heater is employed except that the headers, tube plateand box are in a vertical position and the tubes are arranged invertical alinement. In this form converging angle bars 59 can beprovided for confining the heater assembly in its proper position in theshell. In the ceiling form, the heater assembly is removed through anopening (not shown) provided in the side panel 51 instead of through theend wall as in the floor type of heater.

The present invention also proposes a construction of header and hairpintubes in which the maximum drainage gradient is provided regardless ofwhether the two legs of the hairpin tubes are arranged in a generallyvertical plane (as indicated in Figs. 9 and 12) or in a generallyhorizontal plane (as indicated in Figs. 10 and 13). To accomplish thisthe legs 36 and 37 are spread apart so that they converge toward thebend 43 of the tube. this spreading apart being indicated by thedistance in Figs. 9, 11, 12 and 14, and thereafter the bend 43 of thetube is put in a jig, the leg 36 is bent laterally, (or at right anglesto the plane of the bend 43) in one direction and the leg 37 bent in theopposite direction. This last bending offsets each of the two legs,relative to the plane of the bend, the distance of and offsets the outeror free ends of the two legs the distance of M, as indicated in Figs.10, 11, 13 and 15.

When the hairpin tubes are arranged in a vertical plane, as in Figs. 9and 12, the drainage of condensate presents no problem. The legs arespread apart a distance J-H which is sufiicient to insure that the watercondensing in the upper leg 36 flows freely down that leg. around thevertically disposed bend 43 and down the lower leg 37. Should thegradients not drain properly, the legs of the tubes can obviously bespread further apart.

When, however, the tube sheet is placed on end and the hair-pin tubesarranged in generally horizontal planes, it is obvious that drainagegradients become a distinct problem. Obviously, if the legs 36 and 37-were not ofiset laterally at all, there would be no drainage at allsince both-legs and the bend 43 of each hairpin tube would be arrangedin a horizontal plane. If the legs 36 and 37 are merely oilset laterallyin opposite directions, some drainage gradient will result, but thisdrainage gradient will be very small, the greater part of the distance Mavailable for this gradient being consumed by the bend 43 whichrepresents but a fraction of the length of the tube. Upon now, however,twisting the bend 43 back to its original plane, in accordance with thepresent invention and as shown in the drawings, it is apparent that therelatively short bend 43 is arranged entirely horizontal and has nodrainage gradient and the full distance M is employed as the drainagegradient for the two legs 36 and 37 Thus by holding the bend 43 fast ina jig and bending the legs 36 and 37 laterally outward relative to theplane of the bend, the whole distance M is available for the drainage ofcondensate from the two legs. This would not be true if the bend 43 werenot so held as then the bend 43 would also twist and consume the greaterpart of the gradient distance M. This feature of the invention is, ofcourse, only important when the tubes and header are so placed that thetubes are in generally horizontal planes and has no applicationwhatsoever when the tubes and header are placed so that the tubes are ingenerally vertical planes. 1

The increase of drainage gradients for the legs of hairpin tubesarrangedin a generally horizontal position, as just described, can beclearly observed by manipulating an ordinary hairpin. Take a hairpin andarrange it horizontally and it will be observed that there is nogradient to either of the legs, all parts of the hairpin being arrangedin a horizontal plane. Then, without holding the bend of the hairpin,move one leg up and the other leg down, and it will be observed thatboth legs and the bend are given a gradient but that by far the greaterpart of this gradient is in the bend. Now, holding the legs, twist thebend only of the hairpin back to its original horizontal position andwhile so twisting the bend, observe that the gradient of the legs isgreatly increased, the full gradient formerly consumed by the bend beingtransferred to the legs. Thus the present inven-. tion, when the hairpinis arranged in a generally horizontal plane, increases the gradient ofthe legs at the expense of the bend.

Since, in its application to heating tubing, not only does the bend formonly a small part of the length of the tube but also forms no part ofthe heating surface (the bends being encased, see Fig. 5) it is apparentthat the gradient of the legs is paramount and that the existence ornon-existence of a gradient in the bends is immaterial.

The invention also comprehends the production of a tube and headerheater which can be constructed to provide, within limits, any desiredheat transfer while still maintaining constant face dimensions and aconstant resistance of the tubes to the passage of air.

In unit heaters standardization is desired wherever possible in order toobtain the advantages of mass production. Thus for the full line ofheaters, it is desirable to have but one form and size heater shell orcasing equipped with one form and size of fan and motor. At the sametime the trade demands heaters of different capacity. To meet thedemands of the trade and still obtain standardization in theconstruction of the heater casings, motors and fans, it is apparent thatprovision must be made to supply heating coils of constant facedimensions (each have ing the same dimensions P and C, since otherwisethe dimensions of the casing would have to be altered) and the heatingcoils must also each have the same resistance to the flow of air (sinceotherwise fans and motors of different capacities would have to beprovided) and at the same time each of the coils must be so designed asto provide the desired heat transfer required by the particularcustomer.

The accomplishment of this involves two principles of heating as appliedto air heaters of the present type. The amount of heat transferredvaries directly as the increase in the number of coils or pipes and thefriction of the pipes varies inversely as the square of the velocity ofthe air. In other words the greater the number of pipes, the greater,proportionally, the heat transfer, and by decreasing the velocity of theair, the decrease in the surface friction of the pipes is squared.

For clarity Figs. 15 and 16 have been included, these figurescorresponding to Figs. 11 and 14, respectively, but being simplified toeliminate the complications of hairpin tubing in considering this phaseof the invention. In

the simplified form shown in Figs. 15 and 16,-

the tubes 34a are assumed to be straight tubes running between tubesheets 32-a, the size of the tubes 34a and the face dimensions of theheater (the distances P and C, Fi s. 10 and 13) being identical.

onsidering Fig. 15 which has a single row of pipes 34a, and assumingthat it is desired to increase its heating capacity, it is obvious thatthis could be done by adding another pipe 34w. Assume that this is doneby moving all of the pipes 34a closer together and adding the new pipeat one end of the series. We obtain the desired increase in heatingeffect in the same face dimensions of the heater (the dimensions P andC, Figs. 10 and 13) but the spaces between the pipes 34a, or free areaof the heater is decreased so that the resistance of the coil isincreased and a fan of greater power and capacity must be employed todraw the same amount of air through.

Assume now that instead of doing that, We lengthen the tube sheet, addthe new tube 34-a at the end and at the same time space all of the tubes34a a definite distance further apart. The desired heat increase is nowobtained by the addition of the new pipe 34.a and the resistance of thepipes is maintained constant since we have spaced the pipes far enoughapart to reduce the velocity of the air passing between them just enoughto reduce the friction of the surfaces of these pipes to a point wherethe friction of the new number of pipes is equal to the friction of theold number of pipes. In other words, by spacing the pipes further apartwe square the reduction in their friction to the flow of air to exactlycounteract the added friction of the single pipe we have added.

We still have not achieved our desired object, however, as by carryingout the latter plan we have increased the face dimensions of the heaterboth by adding another pipe 84a and by spacing it and the original pipesfurther apart. Our remaining problem, therefore is to add another pipe34a, space all of the pipes further apart to square the reduction intheir friction to the flow of air to exactly counteract the addedfriction of the single pipe we have added and to do this within the sameface dimensions of the original heater. The obvious solution to thisfinal problem is to stagger the pipes 34a as indicated in Fig. 16.

The heater in Fig. 16 has the same dimensions P and G as that shown inFig. 15 and the frictional resistance of the pipes 34-a to the air flowin Fig. 16 is exactly the same as that in Fig. 15. To make the heatingsurface shown in Fig. 16, we have added four p'pes 34-a to the eightpipes 34-a shown in Fig. 15.

In doing so, we have increased the spaces D and the spaces Y so that theincrease in the sum of the spaces Y squares the reduction in thefriction to the flow of air to exactly counteract the added friction ofthe four pipes 34a which we have added. At the same time we have notincreased the face dimensions (G and P, Figs. 10 and 13) by the simpleexpedient of staggering the pipes 34ca to secure both an increase intheir number and an increase in their zig-zag spacing. We therefore haveachieved our desired object, namely, an increase in heat transferwithout increase in the resistance of the pipes and without increase inthe face dimensions of the heater.

Obviously with the new surface, the sum of the distances Y must begreater than the sum of the distances D since the distances Y mustdetermine the resistance of the surface and not the distances D. It isalso apparent that the invention can be carried out with hairpin tubesas illustrated by Figs. 11 and 14 which correspond to Figs. 15 and 16.

As a whole this invention provides a simple and inexpensive heater inwhich a rise in temperature of the air delivered can be secured byproviding a coil made as described without altering the face area ordimensions of the same or without increasing the resistance of the coilto the flow of air. The coil also is arranged to provide an adequategradient for the condensate in either a vertical or horizontal position,its non-heating surfaces are adequately boxed in, the coils are securelyheld in fixed relation to each other and form a rigid structure, and thecoil is so supported in the shell as to be readily removable therefrom.Furthermore, by making the entire assembly of aluminum, not only is alight and non-corroding structure provided, but the same is more easilyfabricated and can be produced at a lower cost.

I claim as my invention:

1. A hairpin tube of the character described wherein the legs of thetube are offset laterally in opposite directions relative to the planeof the bend, thereby to provide a relatively large gradient in said legswhen said bend is horizontally disposed.

2. A hairpin tube of the character described comprising a bend anddivergent legls,

- said legs being laterally offset substantia perpendicular relative tothe plane of said bend, thereby to provide a relatively large adient insaid legs when said bend is horizontally disposed.

3. A coil assembly comprising headers, means connecting the legs of aseries of hairpin tubes with said headers, the centers of the bends ofsaid tubes lying in the same straight line and the legs of each of saidtubes being divergent from the plane of said bend, the correspondinglegs of the tubes being uniformly laterally offset in one directionrelative to said planes and the other legs being uniformly laterallyofiset relative to said planes and in the opposite direction relative tosaid first named legs whereby when said bends are horizontally disposeda ielatively large gradient is provided in said egs.

4. A coil assembly comprising a tube plate,

hairpin tubes extending laterally from oneface of said tube plate, thelegs of said tubes extending through said tube plate, headers secured tothe opposite side of said tube plate and enclosing the ends of saidlegs, the centers of the bends of said tubes lying in the same straightline and the legs of each of said tubes being divergent from the planeof said bend, the corresponding legs of each tube being uniformlylaterally offset in one direction relative to said planes and the otherlegs bein uniformly laterally ofi- I:

a second independent'header welded at its edges to said tube plate andenclosing the ends of the other legs of said tubes, and an inlet andoutlet for said headers.

6. A coil assembly including a series of hair pin tubes, headersconnected with the legs of said tubes, and a plate at the outer ends ofsaid tubes and holding said tube ends in fixed spaced relation.

7. A coil assembly including a series of hairpin tubes, headersconnected with the legs of said tubes, and a slotted plate engaging theouter ends of said tubes and holding said tube ends in fixed spacedrelation, the bends of said tubes extending through said slots and beingwelded to said plate.

8. A heater including a shell, a coil assembly in said shell, and meansfor forcin air through said shell and over said coi, said coil assemblyincluding a tube plate, hairpin tubes extending outwardlyfrorn one sideof said tube plate, headers arranged on the opposite side of said tubeplate, and enclosing the ends of said tubes, an inlet and outletfor saidheaders, a slotted plate at the outer ends of said hairpin tubes, thebends of said tubes extending through said slots and being welded tosaid slotted plate,

a wall projecting outwardly from said tube plate and preventing the flowof air past said headers, and a second wall projecting outwardly fromsaid slotted plate and preventing the flow of air past said bends.

9. A heater including a shell, a coil assembly in said shell, and meansfor forcing air through said shell and'over said coil, said coilassembly including a tube plate, hairpin tubes extending outwardly fromone side of said tube plate, headers arranged on the opposite side ofsaid tube plate, and enclosing the ends of said tubes, an inlet andoutlet for said headers, a slotted plate at the outer ends of saidhairpin tubes, the bends of said tubes extending through said slots andbeing welded to said slotted plate, a wall projecting outwardly fromsaid tube plate and preventing the flow of air past said headers, a

tube plate and slotted plate and the walls connected thereto toremovably support said coil assembly in said shell.

10. A heater including a shell, a coil assembly in said shell, and meansfor forcing air through said shell and over said coil, said coilassembly including a tube plate, hairpin tubes extending outwardly fromone side of said tube plate, headers arranged on the opposite side ofsaid tube plate, and enclosing the ends of said tubes, an inlet andoutlet for said headers, a slotted plate at the outer ends of saidhairpin tubes, the bends of said tubes extending through said slots andbeing welded to said slotted plate, a wall projecting outwardly fromsaid tube plate and preventing the flow of air past said headers, and asecond'wall projecting outwardly from said slotted plate and preventingthe flow of air past said bends, and means for removably supporting saidcoil assembly in said shell comprising an opening provided in one wallof said shell, and angle bars secured to opposite sides of said shelland terminating at said opening, said angle bars being adapted tosupport the said tube plate and slotted plate together with theirassociated walls at their lower corners to permit said assembly to heslid out through said opening on said angle bars.

11. A coil assembly including a series of hairpin tubes, tube platemeans welded to the ends of said tubes, headers welded to the tube platemeans and enclosing the ends of said tubes, an inlet and outlet for saidheaders, and a plate welded to the other ends of said tubes, saidelements being composed of aluminum.

12. A coil assembly including a series of hairpin tubes, tube platemeans welded to the ends of said tubes, headers welded to the tube platemeans and enclosing the ends of said tubes, an inlet and outlet for saidheaders, a wall welded to said tube plate means and projecting laterallytherefrom to prevent the flow of air past said headers, a slotted plateat the outer ends of said tubes, the bends of said tubes being receivedin said slots and being welded to said slotted plate, and a wall weldedto said slotted plate and projecting laterally therefrom to prevent theflow of air past said bends, said elements being composed of aluminum.

13. A heater composed of two adjacent series of spaced heating membersthrough which series the fluid to be heated successively passes, themembers of one series being staggered with reference to the members ofthe other series and the ratio between the number of heating elementsand the staggered free area of the/members being such that the amount ofstaggered free area is increased substantially as the square root of theincrease in the number of heating members and vice versa.

14. A heater composed of a series of spaced heating members throughwhich the fluid to be heated passes, some of said members being arrangedin advance of the others with reference to the fluid flow, and the ratiobetween the number of heating elements and said effective free areabeing such that the amount of effective free area is varied uniformlyand directly substantially as the square root of the variation in thenumber of heating elements.

15. A heater composed of at least two series of spaced heating members,the members of each series being arranged substantially in the sameplane and the two series being arranged substantially parallel and inproximity to one another with the members of one series staggered wit-hreference to the other series so that a planar free area exists betweenthe members of each series and a zig-zag free area exists between themembers of the two series, said elements being also so arranged that theplanar free area exceeds the zig-zag free area, whereby the zig-zag freearea forms substantially the only area affecting the resistance of theheater to the flow of the fluid to be heated therethrough and saidzig-zag free area being accurately determined to provide the exactresistance desired.

16. A method of making heaters for air or the like which consists ofmounting two series of heating elements in spaced relation with themembers of one series staggered with relation to the members of theother series and in such manner that the ratio between the number ofheating elements and the staggered free area between the members is suchthat the amount of said staggered free area varies substantially as thesquare root of the variation in the number of heating members.

In testimony whereof I hereby aflix my signature.

CLAUDE A. BULKELEY.

